The present invention relates both to a milling process and to a milling tool for the cutting of workpieces with flat or curved surfaces.
Of the known milling processes for working curved surfaces, the most frequently used process involves passing a shank end mill with a spherical end, termed a spherical end mill, and using three translatory motion axes at right angels to one another on planar or spatially curved paths over the workpiece. The spatial distance between the sphere center and the surface to be produced is kept constant.
The advantage of this process is it can be used without difficulty on standard milling machines with a corresponding control system and corresponding feed drives. There are also few collision problems. Collision problems arise when certain positions of the milling cutter and its spindle occur during milling when the milling cutter or parts of the milling machine must penetrate the workpiece at undesired points. In particular, these problems arise at re-entrant contours and surface working must be interrupted for avoiding the same. In addition, relative inexpensive tools can be used in this process.
However, in this known process, the adaption between the milling tool and the workpiece is poor. As a result a very large number of milling paths are needed to attain a specific roughness for a workpiece. Moreover, as a function of the construction of the work surface often unfavorable cutting conditions exist.
Another known milling process is circumferential milling with cylindrical or disc-shaped milling tools. In the case of circumferential milling with cylindrical milling tools, the advance or feed takes place in a direction which is substantially at right angles to the milling tool axis. As a result of the collision problems which occur, this process is limited to a few special uses, such as producing turbine blades which are purely cylindrical or the working of certain edges. Attempts are made to bring about optimum adaption between the tool flank or edge and the work surface. In certain applications, this adaption can be improved by the use of special form cutters.
In the case of circumferential milling with disc-shaped milling tools the feed takes place essentially parallel or at right angles to the milling cutter axis. This process has the advantage that more powerful milling tools can be used and these tools will exhibit a high cutting capacity. However, once again collision problems occur, particularly in the case of concave shapes, due both to the position of the cutting spindle (the milling cutter axis is substantially parallel to the work surface or is only slightly inclined with respect thereto, namely max. approx. 30.degree.) and because the diameter of the milling tool cannot be reduced below a certain amount, approximately 25 mm, so that in the case of concave portions with smaller curvature diameters, a collision risk exists. It is also disadvantageous that the milling force acts on the work surface under a relatively steep angle, which can lead to sagging, vibrations and chattering in the case of thin not very dimensionally stiff workpieces, such as turbine blades. Particularly as a result of the aforementioned collision problems, this process is also mainly used for special applications, in particular the machining of turbine blades.
Milling processes referred to as camber milling are also known. The camber is considered to be the inclination of the milling cutter axis to the surface normal at the contact point between the milling cutter and the work surface. The direction of the milling cutter axis is described by angles .gamma. and .delta.. Angle .gamma. is formed by the milling cutter axis and the surface normal and is called the camber angle. The second angle .delta. is between the projection of the milling cutter axis on the surface, called the "camber" direction; and the feed direction.
In such a known process of the same Applicant (Swiss patent application No. 6727/83) for the machining of blade-like workpieces, the blade edges are shaped by circumferential milling using a cylindrical milling tool, e.g. a shank end mill, along the blade edge generatrixes, while the wide sides of the blades are worked with the face of the shank end mill with camber in interrupted all-round milling, the already shaped blade edges being jumped in rapid travel. Although this process solves the collision problem only slightly less well than in the first mentioned process and also permits better cutting efficiencies to be obtained, considerable time losses occur as a result of the lifted jumping of the blade edges.
In another known camber milling process (German Pat. No. 25 44 612), the blade-like workpiece is machined with a face milling cutter with camber, the feed taking place around the blade. Although here again the collision problems are only solved slightly less well than in the first mentioned process and better cutting efficiencies are obtainable, a relatively large amount of time losses occur in milling round the blade edges.
In camber milling, in which a constant camber angle is maintained, on non-continuous paths (continuous paths can be formed by spiral or helical movements), there is still a considerable time loss on changing from one path to the next, either in the case of only milling in one direction with the return movement taking place with the cutter raised in rapid transit, or in the case of reciprocating milling, in which during each reversal with line jump the milling cutter is raised, the camber angle direction reversed by l80.degree., followed by the lowering of the milling cutter onto the workpiece again.
In another known camber milling process (U.S. Pat. No. 4,104,943), milling takes place by using a triaxial milling machine both with a pulling out and with a pushing cut, so that a spatially constant angle is maintained between the milling cutter rotation axis and the milling cutter feeds means or machine axes.
In the case of triaxial camber milling of curved surfaces there is a change to the camber angle, i.e. the angle between the milling cutter axis and the local surface normal changes constantly, so that generally unfavorable cutting conditions exist. This has a particular effect during the pushing cut, which is generally very difficult. At the latest it fails when the milling cutter center is pressed into the material surface.
In particular, normal milling cutters are fundamentally unsuitable for camber milling with a pushing cut, so that this process is scarcely used in practice.